Put a skirt, mother, on at least! (7)
I believe the answer is:
minimum
'least' is the definition.
(similar in meaning)
'put a skirt mother on at' is the wordplay.
'put a skirt' becomes 'mi' (I am not sure about this - if you are sure you should believe this answer much more).
'mother' becomes 'mum' (informal term for mother).
'on' indicates putting letters inside (as in clothing 'on' a person).
'at' becomes 'in' (synonymous in some cases - eg at school, in school).
'mi'+'mum'='mimum'
'mimum' enclosing 'in' is 'MINIMUM'.
Can you help me to learn more?
(Other definitions for minimum that I've seen before include "Nadir" , "Smallest possible quantity" , "Lower limit" , "Lowest point" , "Bottom" .)